This invention relates to a method and an apparatus for recovering polonium-210 from irradiated bismuth using pyrochemical extraction techniques.
Neutron irradiation of bismuth produces polonium-210 according to the reaction .sub.83 Bi.sup.209 + .sub.0 N.sup.1 .fwdarw..sub.83 Bi.sup.210 5.4 days 84.sup.Po .sbsp.1.sbsp.0. Polonium- 210 has a half life of about 138 days and decays by alpha emission to stable lead-206. Because of its short half life and high specific activity, polonium-210 is a valuable isotopic power source.
Various processes have been proposed and tested for the recovery and separation of polonium-210 from irradiated bismuth. In most processes the bismuth is dissolved in an acidic medium, such as acid chloride and the polonium-210 subsequently recovered and purified by precipitation, absorption, or liquid-liquid solvent extraction schemes. Also known is a method for recovering polonium-210 from bismuth by means of a combination of pyrochemical and liquid-liquid solvent extraction steps wherein molten bismuth containing polonium-210 is contacted at 400-500.degree. C in an inert atmosphere with molten caustic material. The polonium is extracted from the molten metal phase to the molten caustic phase. After the phases are separated and the caustic phase dissolved in nitric acid, the polonium can be recovered by means of liquid-liquid solvent extraction such as treatment with dibutylbutylphosphonate. The prior art methods provide experimental processes for polonium separation but do not disclose an apparatus suitable for extracting large amounts of polonium-210 from kilogram quantities of irradiated bismuth.
The prior art procedures are reported in I. and E.C. Process Design and Development, Vol. 7, pp. 149-150, 1968 and Vol. 8, pp. 508-515, 1969; U.S. Pat. No. 3,463,739; U.S. AEC Report MLM 1661, Apr. 15, 1970; Nuclear Sci. and Eng., Vol. 35, pp. 159-164, 1969 AEC Report D -- 4146, MLM - 915, Aug. 1, 1953; and J. Inorg. Nucl. Chem. Vol. 29, pp. 848-853, 1967.
In order to successfully extract polonium-210 from irradiated bismuth metal, certain processing conditions must be satisfied:
1. oxygen must be excluded from the system to avoid oxidation of polonium-210 to the volatile oxide and to avoid the formation of bismuth oxide which will collect at the bismuth-caustic interface and interfere with polonium transfer from one phase to the other; PA1 2. the bismuth-caustic phases must be intimately stirred or mixed to establish equilibrium conditions between the phases in a minimal amount of time; and PA1 3. The molten bismuth must be contacted with sufficient molten caustic to bring about the transfer of essentially all the polonium from the bismuth to the caustic. The third condition can be accomplished by contacting one batch of bismuth with one large batch of caustic or by contacting both phases in a true counterflow arrangement or by contacting a batch of molten bismuth with several successive small batches of molten caustic material. The apparatus disclosed in this invention satisfies the conditions stipulated above, and in addition, requires fewer processing steps for use than currently used apparatus as well as avoiding the use of chlorides.